This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-055529, filed Feb. 28, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a data encoding method, apparatus, and storage medium. The invention is relates to an apparatus which records digital data onto a storage medium, such as an optical disk, or reproduces the recorded data. The invention further relates to a data encoding method and apparatus suitable for the conversion of digital data into bit sequences to be recorded onto a storage medium, and to a storage medium on which data is to be recorded or has been recorded.
2. Description of the Related Art
To record digital data onto an optical disk or the like, bit sequences of digital data are required to have various characteristics. The bit sequences recorded on a playback-only optical disk are expressed as bumps or pits on the reflecting surface. In a recordable magnet-optical disk, recorded bit sequences are expressed as sequences of magnetized marks. In a phase change disk, recorded bit sequences are expressed as sequences of marks with optical constants.
The disk manufacturing processes are required to be easy. The optical characteristics in reading with a laser beam are needed to be good. The characteristics in reproducing the digital data from the read-out signal must also be good. Therefore, it is necessary to convert the recording data into bit sequences that satisfy the above requirements. That is, an encoding process must be carried out.
When the recorded minimum pit length is small, if the optical characteristics deteriorate, the output of the reproduced signal decreases significantly. Thus, it is desirable that the minimum pit length should be great. Conversely, if the maximum pit length is great, the number of inversions of the reproduced signal decreases. This makes the timing for reproducing the clock unstable, making jitters in the clock larger. As a result, code errors in the reproduced signal are liable to occur. Therefore, it is desirable that the maximum pit length should be small.
It is also important that the direct-current (DC) component and low-frequency components of the recording signal are small. These signal components might affect the tracking servo that traces the track formed on the optical disk. To read the signal accurately, it is necessary to suppress those signal components. Another requirement is that the sensing window width should be wide. If the original data was converted into many bits and recording took place, phase allowance in time during sensing would become smaller, even when the pit length condition was satisfied. At the same time, the reproducing clock frequency would become higher.
One encoding method taking these conditions into account is the 8/14 encoding method disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 6-284015. The 8/14 encoding method is an encoding method of converting 8-bit data into 14-bit code.
In this method, a plurality of tables for code conversion are used and switching is done between the tables in such a manner that the value (DSV: digital sum value) accumulated, taking bit xe2x80x9c0xe2x80x9d as xe2x80x9cxe2x88x921xe2x80x9d and bit xe2x80x9c1xe2x80x9d as xe2x80x9c+1xe2x80x9d in a recording bit sequence, becomes smaller. This makes it possible to suppress the direct-current component and low-frequency components sufficiently.
On the other hand, since the number of bits after the encoding increases to 14/8 times the number of bits in the original data, this makes the sensing window width smaller and increases the clock frequency at the same rate. In recent years, digital recording apparatuses have been required to have higher data transfer speeds. Increases in the data recording and reproducing speeds increase the clock frequency, which might be an obstacle to configuring a signal processing circuit.
Another encoding method has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 56-149152. In the encoding-method, the data is converted into a code containing as many as 1.5 times the number of bits in the data. Since the number of consecutive bits xe2x80x9c0sxe2x80x9d between bits xe2x80x9c1sxe2x80x9d is one or more and seven or less, this type of method is generally called (1, 7) RLL coding.
This method is characterized in that a signal processing circuit can be configured using a small circuit and a relatively low clock frequency. Since this method does not manage the DSV at all, as in the 8/14 method, the direct-current component and low-frequency components cannot be suppressed. Thus, in this method, the tracking performance might be affected. To avoid this phenomenon, it is necessary to insert adjustment bits in addition to the recording data to make the DSV smaller. When such adjustment bits are inserted, the number of redundant bits on the medium increases. This causes a new problem: the storage capacity of the original data decreases. There is another problem: since variable-length coding includes the conversion of 2 bits into 3 bits and the conversion of 4 bits into 6 bits, bit errors are liable to propagate.
As described above, in the conventional encoding methods, suppressing the low-frequency components permits the clock frequency to rise, whereas decreasing the clock frequency to a low level makes it impossible to suppress the low frequency components. There is still another problem: adding adjustment bits increases the number of redundant bits, which decreases the storage capacity of the original data on a storage medium.
It is an object of the present invention to provide a data encoding method and a data encoding apparatus which need not raise the recording clock frequency too much and can suppress the low-frequency components without using adjustment bits, and a storage medium.
According to an aspect of the present invention, there is provided a data encoding method of converting an L-bit data word into an xcex1L-bit code word, comprising: dividing the L-bit data word into an M-bit data word and an N-bit data word (Mxe2x89xa7N); converting the M-bit data word into a xcex2M-bit code word by reference to a first conversion table; converting the N-bit data word into a xcex3N-bit code word by reference to a second conversion table; and connecting the xcex2M-bit code word to the xcex3N-bit code word to convert into the xcex1L-bit code word.
Whether to select either the first or the second conversion table is determined by a combination of the M-bit data word and the N-bit data word.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.